The inkjet printing method is a printing method in which very fine liquid droplets of ink are jetted from an inkjet head and deposited onto a printing target substrate, thus forming images or text on the substrate, and has the characteristic feature of not requiring a printing plate. The electrophotographic method is also widely recognized as a printing method that does not require a printing plate. However, compared with the electrophotographic method, the inkjet printing method is superior from various perspectives, including the initial cost of the apparatus, the running costs during printing, the apparatus size, and the high-speed printing characteristics.
Conventionally, a wide variety of inks have been used for the inks used in inkjet printing methods, including solvent-based inks, aqueous inks and oil-based inks. However, in recent years, the demand for active energy ray-curable inkjet inks has been increasing. This is because active energy ray-curable inkjet inks offer the advantages of also being applicable to non-absorbent substrates such as plastics and glass, and being able to reduce environmental impact by reducing the amount of solvent volatilization. Particularly in the case of inkjet printing for industrial applications, additional factors such as the water resistance of the printed items, the energy required for drying the ink and the adhesion of ink components to the head as a result of ink drying are expected to accelerate the replacement of solvent-based inks and aqueous inks with active energy ray-curable inkjet inks.
Furthermore, in recent years, as the performance of inkjet heads has improved, the application of inkjet printing methods within other known printing markets such as offset printing has also become more likely. In existing printing markets, productivity is extremely important. However, in the signage market, multi-pass printing methods are used, and with this type of printing method, achieving the type of productivity desired in existing printing markets is difficult. Accordingly, in order to enable an inkjet printing method to be used in existing printing markets, a single pass printing method capable of high-speed printing is preferably used.
On the other hand, the printed items from existing printing markets typified by offset printing are printed using a multitude of special colors (such as orange, red, green, blue and violet), and therefore offer the advantage of having excellent color reproducibility. Accordingly, in order to enable the application of an inkjet printing method within an existing printing market, achieving excellent color reproducibility is very important. Moreover, it is desirable that the obtained printed items do not undergo discoloration during storage.
In the case of inkjet inks, Patent Documents 1 to 3 disclose that because the color reproducibility of the formed images varies considerably depending on the degree of color reproduction potential of the three colors of yellow, magenta and cyan, pigment selection is an extremely important factor.
In Patent Documents 1 and 2, the desired color gamut is ensured by appropriate pigment selection, but these documents relate to aqueous inkjet inks. The present invention relates to non-aqueous inkjet inks, and in non-aqueous inkjet inks, because the dispersion solvent medium is generally a non-aqueous solvent such as a polymerizable monomer, pigment dispersion in the solvent medium tends to be extremely difficult, meaning further improvements are desirable.
In Patent Document 3, a method for improving the color reproducibility in active energy ray-curable inkjet inks is investigated, in which a so-called solid solution containing unsubstituted quinacridone and 2,9-dichloroquinacridone mixed together within the same crystal is selected as the magenta pigment. However, quinacridone pigments have a problem in that they tend to suffer from inferior color development. Moreover, pigment dispersions using a solid solution have poor storage stability. In addition, active energy ray-curable inkjet inks that use a solid solution tend to exhibit slower curing rates, and therefore considering the current trend toward high-speed printing, the use of quinacridone solid solutions could be considered undesirable.
Furthermore, in order to reproduce the type of color development achievable with offset printing, using an ink set containing a plurality of special colors is simple, but this results in an increase in the apparatus costs due to an increase in the number of heads, which diminishes the low-cost effect that is one of the benefits of inkjet printing.